Bone-calcified tissue is a highly complex and dynamic organ in the human body. Regulated chemical components within the bone-forming cells control the extracellular chemical activities which produce the calcified bone materials Escarot-Charrier, et al., J. Cell. Biol. (1983), 96, 639-643; Sudo, et al., J. Cell. Biol. (1983), 96, 191-198; Stein et al., Proc. Natl. Acad. Sci. USA (1988)). The two primary chemical components of bone are inorganic calcium phosphate solids and organic collagen matrix. Several different types of calcium phosphate minerals exist, including calcium hydroxyapatite, tricalcium phosphate, octacalcium phosphate, etc. Only calcium hydroxyapatite has been established as a major mineral constituent in human bone. The size of the hydroxyapatite crystals found in bone are extremely small, on the order of several hundred .ANG. wide and several microns long. These tiny crystals lend their unique characteristics to the rigidity of the bone tissue.
Collagen is a ductile organic bio-polymer that provides molecular binding sites for the calcium phosphate minerals and provides the flexibility to the overall mechanical property of the bone. Collagen is a major protein component in the human body and is found in skin, cartilage and tendon.
It is the combination of the calcium phosphate minerals and the collagen, in combination with other minor components, which provides the unique structural, chemical and biological properties of bone tissue.
For the most part, bone or bony implants involve non-biological materials, primarily metal alloys, such as titanium alloys, stainless steel and cobalt chromium alloys. These materials provide for superior mechanical properties, such as fracture toughness, load properties and ability to maintain a good stress-strain relationship. Their use is predicated on the inability to produce bony structures which can be successfully introduced to replace diseased, fractured or otherwise non-functional bony structure present in the host.
Greater than fifty percent of the total orthopaedic surgery performed on patients today fails during the first ten years. The failure is primarily due to the lack of biocompatibility between the prosthetic material and the naturally occurring bone tissue. The inability for bony tissue to provide a strong bond with the prosthetic device causes several clinical complications. The most serious complication is a tissue rejection process that occurs along the surface of the prosthetic material which results in bone resorption. The bone resorption process creates a "gap" between the prosthesis and the adjacent bone surface. This gap facilitates the movement of the prosthesis, causing severe pain to the post-operative patient and the ultimate failure of the implant.
Despite the large amount of effort which has already been expended in trying to solve this problem, the problem has remained substantially intractable. Efforts have been primarily directed to using new methods of coating orthopaedic prostheses with calcium phosphate minerals. There has been continued concern about a natural integration between new bone growth and the prosthesis. There is, therefore, substantial interest in finding new techniques which will enhance the integration between new bone growth and prostheses and provide for extended useful periods without the problems associated with prostheses today.